Making bamboo fins - I didn't plan on them cupping in the process

Hey, can anyone help me. I purchased some of the bamboo sheets from Greenlight and was set to make some fins. I cut my templates out, making sure I cut the fins with the grain running the same direction of the fin rake, a set of keels and a set of quad fins. I used my ban saw and starting in on sanding them down. Being the garage hacker that I am, I have a full time job to support my obsession, so I only have small windows of time to practice "the craft". As I get closer to finishing my front fins for the quad set, I'm noticing them bent and/or slightly twisting a bit? I picked up my keels (cut out but not sanded down yet) and see they too have pulled a bit, they aren't straight any more? I piled like 20lbs of hammers and weights on the keels and let them sit for a few days, but they are still bent. Now I live in SD and my garage is dry, is this just a feature of bamboo? I've never had this happen using marine-grade plywood? HELP!!!!! DO I have to trash these bad boys and cut, sand and glass another set all in one sitting??? Can I glass them hoping they'll somehow pull straight?

Hey delasol,  it sounds like the layup of the blank may not have been perfectly balanced regarding the ‘grain’. Here’s a typical grain pattern for laying up a wood panel that will resist cupping or warping:

Obviously bamboo does not have quite the same endgrain pattern, but it does vary in density, flexibility, hardness, etc. from the outer to inner surface of the culm.

 

 

Not a big deal regarding those fins though.  I glue up my own blanks in a very humid part of the country and have to anticipate this problem with every layup I put together.  Usually the grain orientation will take care of warping.  In the rare instances that it doesn’t, I’ll glass the flat side of  a blank with pressure on it to keep it flat until the resin cures.  It is likely that the cupping or warp will be mostly reduced, but not completely gone.  However, that can change once the strength of the wood (and its warping tendency) is reduced by foiling the opposite side.  Most times after foiling a slightly warped blank that was glassed flat on the opposite side, the tension in the glass side will take over and flatten it out. 

 

Hope that helps man.  If not, you’ve got yourself a set of fins with prefabbed ‘vector’ foils in them, maybe even some high tech semi-helicals.  Score!

Camplus, thanks for the info. I thought I might be able to reduce the cupping if I glass one side and weigh them down once they are not tacky anymore. Yeah, I thought too, just maybe I could use these if they cup the same, fat chance! I may trim them down and use as small side bits. Live and learn...Thx again.

the photos of the fin making for the gene cooper BIG GUN

were quite telling .

In these photos

the step of running

the finblank,glassed and cured,

through the dimensional sander 

to me was a conceptual breakthrough

as well as the seed pearl of new tool lust…

 

when the fin is at the

‘‘now let us foil stage’’

it helps thet the ‘fin blank’

has a dimensional regularity

in the past I have always

faked it with my grinder

gettin’ it kinda flat and 

just asuming the good

enough attitude…

 

the cupping dimensional stuff -living with it tude

is a viable option,but once seen, the dimensional sander

is an eclipse of the entire fin building solar system…

…ambrose…

is there a dimensional sander in the house?

under the xmas tree?

on e-bay?

at the dump?(for salvage like bad cord?)

mebe a yard sale?Hey if it’s not glassed yet

just put a piece of sand paper on a sheet of glass

an san it flat !

I’ goin’ craiks liss ri now!

     Howzit delasol, Are you using epoxy or polyester resin for the fin, makes a difference which it is. First off I would do both sides at the same time. Both epoxy and poly shrink as they cure and that could be part of the problem. Poly shrinks some where around 6% and epoxy 3%, these are just estimates and could be more or less.But if you do one side and it shrinks, when you do the other side the cured side may not let it go back to how they were to start. As I was writting this you coud turn a mistake into a good thing. If the fin is curved so you could use them and the curve be set facing outwards. In effect you would have a slight curved Wood fin. Could be a first since the origianl ones are mistakes and not meant to be that way. Glass the side that will be facing out is the way to way you have some control on the process. I'm sure somebody has done it on purpose and I know FCS has them but those are really curved and inwards where these would be more like flared,just 2 more cents. Aloha,Kokua